cosmos-sdk: gov/v1beta1/proposal doesn't handle correctly new SoftwareUpgradeProposal

Summary of Bug

ping.pub and Kepler explorer crash when opening a governance page, when there is a new SoftwareUpgradeProposal (submitted as a new tx gov submit-proposal) as well as any new proposal gov message. The exception in the browser console is: TypeError: t.p.contents is null. Example. Probably other explorers (mintscan…) will crash as well.

The reason is that gov/v1beta1/proposals endpoint doesn’t set Content for the new Proposal types. The problem is that today, explorers use gov/v1beta1/proposals and expect Proposal.Content.

Ecosystem bugs:

• It makes one of the most important features: gov almost unusable without a frontend.
• Without that, literally nobody should use new gov system (they should be marked experimental, until ecosystem adoption is achieved, rather then deprecating the “working” solutions).
• We need an ecosystem push for tooling update.

Version

v0.46.7 I guess, v0.47-alpha2 has same issue.

Steps to Reproduce

1. Start simd

2. Make proposal upgrade:

    simd tx gov submit-proposal p.json \
      --from w1 --chain-id $CHAINID  -y --keyring-backend test --home $APP_HOME --fees 5000$DENOM -b block
   

3. wait until upgrade, and start new binary with compiled upgrade plan, and API server enabled

4. Open ping pug gov page - it will break

5. You won’t be able to see details of any new gov proposal, until explorer will start supporting gov/v1

Open 127.0.0.1:1317/cosmos/gov/v1/proposals/1, you will see correct response:

{
  "proposal": {
    "id": "1",
    "messages": [
      {
        "@type": "/cosmos.upgrade.v1beta1.MsgSoftwareUpgrade",
        "authority": "cosmos10d07y265gmmuvt4z0w9aw880jnsr700j6zn9kn",
        "plan": {
          "name": "v3.0.0",
          "time": "0001-01-01T00:00:00Z",
          "height": "45",
          "info": "",
          "upgraded_client_state": null
        }
      }
    ],
    "status": "PROPOSAL_STATUS_PASSED",
    "final_tally_result": {
      "yes_count": "1090000000000",
      "abstain_count": "0",
      "no_count": "0",
      "no_with_veto_count": "0"
    },
    "submit_time": "2022-12-15T23:20:00.636433951Z",
    "deposit_end_time": "2022-12-15T23:21:00.636433951Z",
    "total_deposit": [
      {
        "denom": "stake",
        "amount": "10000000"
      }
    ],
    "voting_start_time": "2022-12-15T23:20:00.636433951Z",
    "voting_end_time": "2022-12-15T23:20:20.636433951Z",
    "metadata": ""
  }
}

open 127.0.0.1:1317/cosmos/gov/v1beta1/proposals/1, you will see response with null content

{
  "proposal": {
    "proposal_id": "1",
    "content": null,
    "status": "PROPOSAL_STATUS_PASSED",
    "final_tally_result": {
      "yes": "1090000000000",
      "abstain": "0",
      "no": "0",
      "no_with_veto": "0"
    },
    "submit_time": "2022-12-15T23:20:00.636433951Z",
    "deposit_end_time": "2022-12-15T23:21:00.636433951Z",
    "total_deposit": [
      {
        "denom": "stake",
        "amount": "10000000"
      }
    ],
    "voting_start_time": "2022-12-15T23:20:00.636433951Z",
    "voting_end_time": "2022-12-15T23:20:20.636433951Z"
  }
}